1. Field of the Invention
This invention relates to dipeptide crystals having a sweet flavor, as well as to a process for their production. The present invention also relates to tablets containing the sweet dipeptide crystals which have excellent storage stability, and also to a process for the production thereof.
2. Description of the Prior Art
.alpha.-L-Aspartyl-L-phenylalanine methyl ester (hereinafter referred to as AP), known popularly as Aspartame, is a low calorie sweetener having a sweetening quality similar to sucrose which is pleasant. AP has attracted considerable attention lately in this regard. However, when the practical use of this substance is considered as a sweetener, several properties of AP such as low solubility and dispersibility present serious disadvantages.
Further, AP is relatively unstable to heat and when it is heated it is easily converted into a diketopiperadine and loses sweetness. For example, when a solution of AP is heated to 80.degree. C. for about 5 hours, approximately 20% of the AP is decomposed. Hence, even in the production of AP, the production steps must be monitored so as not to exceed 60.degree. C.
To date, two types of AP crystals have been known to exist. These crystals are generally known as I type crystals and more specifically as I.sub.A or I.sub.B type crystals. Both varieties of I type crystals are very hygroscopic resulting in coloration and decomposition during storage. Additionally, the pelletizability and fluidizability of powders containing these I type crystals is very poor.
The hygroscopic nature of these crystals also present problems when attempting to formulate a usable AP tablet. For example, in the case of effervescent tablets, either moisture absorbed or generated internally by chemically reaction of the effervescing agents (basic) and neutralizing agents (acidic) can cause adhesion between the individual tablets or between the tablets and the container. Further, the reaction of effervescing and neutralizing agents reduces tablet effervescence during storage and hence, the available tablet effervescence upon use is diminished. Due to the reduction in effervescence, the solubility of the tablet is greatly reduced.
Thus, a need clearly continues to exist for crystals of AP having excellent storage stability and which are very stable even in the presence of excipients, effervescing agents or neutralizing agents. Further, a need continues to exist for AP tablets which exhibit a reduced rate of neutralization reaction thus maintaining tablet effervescence and high tablet solubility.